The Specific Aim of the project is to improve the survival of cultured endocrine allografts by (1) Modifying culture conditions to increase the selective destruction of passenger leucocytes without damage to parenchymal cells, (2) Determining the mechanism of graft rejection, and (3) Determining the mechanism of the active tolerance that develops in long term recipients of cultured allografts. The first aim will be accomplished by first reducing the minimum oxygen pressure required for abrogation of rejection through methods that improve oxygen diffusion, and then by increasing the maximum oxygen pressure tolerated with antioxidants. The second aim will be approached by experiments that will (1) determine if MHC-incompatible grafts are selectively spared or rejected, (2) determine if a correlation exists between the suppression of cytotoxic (CTL) production in vivo and the protection against graft rejection, and (3) demonstrate whether the DNA fragments characteristic of CTL killing that appear in rejecting grafts come from host or graft cells. The third aim will be studied by the approaches now under development that permit a quantitative measurement of tolerance both in vitro and in vivo. In vitro studies are based on the measurement of CTL responses with and without the addition of IL-2 and the addition to cell cultures of immunoglobulin from tolerant animals. In vivo studies are based on an analysis of the suppressed CTL response of tolerant animals to the I.P. injection of allogeneic tumor cells.